For thermal sensitive recording systems utilising changes in physical properties and chemical reactivity of a material corresponding to an input of thermal energy, many proposals have been already made. Among them, a thermal sensitive colouring recording system and a thermal sensitive transfer recording system have been actively developed in recent years. The latter includes thermal melt transfer systems, which have found applications in printers, facsimile machines, and copiers for example, because of their capability of recording on plain paper, their superiority of light fastness, stability and shelf life of the recorded image, and their high reliability due to simplicity of its recording mechanism, etc.
Unfortunately, these known machines are not without their problems. Thus when a transfer image with a high precision and a high resolution is required for example, the thermal head becomes expensive. There are problems and difficulties in making a heater element small, and in controlling the heat generated by the heater element. Moreover, when an image with a large size is to be printed, it is extremely difficult to make a line head which operates uniformly throughout its length. If laser beam is used as the heat source, a spot diameter can be focused in a small size and thus a transferred image with a high precision and a high resolution can be obtained.
In the case of a beam source of a compact and a relatively low price, such as a semiconductor laser for example, the speed was not always sufficient because of the low output of the source. In order to utilise the energy of the semiconductor laser more effectively, it has been proposed to incorporate a substance (hereinafter, it is simply called an "opto-thermal conversion substance") which has a capability of an opto-thermal conversion to absorb a beam of 700-900 nm, a wavelength region of the semiconductor laser, in a colour material layer (Japan patent laid-open Tokukai Sho 63-319192). However, in case of the thermal melt transfer recording system, since an ink layer itself is transferred to a receiving paper, there was a drawback that the colour purity of an image was deteriorated by a colour of the opto-thermal conversion substance.
It has also been proposed to prepare an independent opt-thermal conversion layer to prevent scattering and transferring of ash by igniting a binder of a layer containing the opto-thermal conversion substance in the dye diffusion type transfer recording system (Japan patent laid-open Tokukai Hei 2-252580) or to make a constant output of the laser beam to each colour ink in the thermal melt transfer colour recording system (Japan patent laid-open Tokukai Sho 63-22684). However in the thermal melt colour transfer recording system using a semiconductor laser beam as a heat source, it was strongly demanded to make an ink sheet by which a practical recording speed could be obtained.
As described above, for a colour printer for office use for which downsizing and low power consumption are required, it is desirable to obtain a practical recording speed while using a relatively low output energy source such as a semiconductor laser, and for this purpose it is indispensable to improve the efficiency of the opto-thermal conversion layer. This invention offers an ink sheet for the thermal melt transfer type colour recording to give a practical recording speed at a low output by providing a more effective opto-thermal conversion layer.